Since a cover letter will already include the highlights of the CV that are relevant to that particular company and job advert, is it necessary to tweak the CV as well?
Ideally, yes, you would customize the resume for each position so that it emphasizes the particular skills and experience that best matches up with the job description. If you're doing it right, that sort of customization can make it more likely that your resume stands out from the crush of resumes the company is reviewing.
In the real world, however, that doesn't always work out so well. Customizing your resume can introduce typos and it can lead people to introduce phrases that aren't as polished as the rest of the document for example which can hurt your chances. And many people aren't particularly good at marketing themselves which may lead them to emphasize the wrong things or to de-emphasize their best attributes just because they didn't show up in the text of the job description. Plus, it's a great deal of work to customize resumes for every job so that necessarily takes away from other things that you can be doing to land a job. You need to balance the costs and the benefits.
Commonly, people that expect to apply for different types of jobs will maintain different versions of their resume that are targeted to each type of job. A developer might have one resume that focuses more on their front-end skills and another that focuses on their back-end skills depending on the job. Someone else might have one resume that focuses on their waitressing skills and another that focuses on their front desk experience. That achieves much of the benefit of customizing the resume at much less cost than customizing the resume every time.
People that are applying for jobs that hundreds of other people are also going to apply for probably need to send out far more resumes than someone that is applying for a position that relatively few people are qualified for. It probably makes more sense to spend the time customizing the resume in the latter case where you know that a human is going to be reviewing them carefully than in the former case where it's very likely that computers are looking for keywords and humans are applying some gross filters like looking for a degree and a certain number of years of experience.
Your CV should be tailored to the job you're applying for, emphasising the skills you possess that the employer is looking for.
I maintain several versions of my CV, tailored towards technical roles, leadership, architecture etc.
Always keep the version you've sent to an employer, so you can re-check it before and interview to ensure you have the same vision in mind.
Most HR systems today use some form of Applicant Tracking System. Cover letters do not get processed in the same way as the resume/CV, so vital information that is only on the cover letter might not be used to qualify you for the position, or it might not be seen by the decision makers (it will be available to be seen, but that is not the same as being seen).
As a candidate, you have to assume that the people initially reviewing your application have many resumes/CVs to get through in a short amount of time. If you make their job easier, you move the odds in your favor. If you make their job harder (by spreading important information across multiple documents instead of consolidating it into one - your CV) you lower your odds of getting through the process.
So, to your question, if it is vital, put it in the CV and use the cover letter to call attention to it.